A New Method for Testing Filtration Efficiency of Mask Materials Under Sneeze-like Pressure
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in vivo 34: 1637-1644 (2020)
doi:10.21873/invivo.11955
A New Method for Testing Filtration Efficiency
of Mask Materials Under Sneeze-like Pressure
LI XIAO1, HIROSHI SAKAGAMI2 and NOBUHIKO MIWA3
1Department of Pharmacology, School of Life Dentistry at Tokyo, The Nippon Dental University, Tokyo, Japan;
2Meikai University Research Institute of Odontology (M-RIO), Saitama, Japan;
3Faculty of Life Sciences, Prefectural University of Hiroshima, Hiroshima, Japan
Abstract. Background: Sneezes produce many pathogen- Severe acute respiratory syndrome coronavirus 2 (SARS-
containing micro-droplets with high velocities of 4.5-50.0 CoV-2) causing respiratory infection, has rapidly spread
m/s. Face masks are believed to protect people from infection worldwide. SARS-CoV-2 is primarily transmitted between
by blocking those droplets. However, current filtration people through respiratory droplets and small particle droplet
efficiency tests can’t evaluate masks under sneeze-like nuclei (aerosols). Coughing and sneezing can produce
pressure. The goal of this study was to establish a method to airflows at high velocities containing countless micro-
evaluate the filtration efficiency of mask materials under droplets. Early studies showed that the velocity of a sneeze
extreme conditions. Materials and Methods: Efficiency of was about 46.0-50.0 m/s whereas recent studies
surgical masks, gauze masks, gauze, cotton, silk, linen and demonstrated that the initial velocity of the micro-droplets
tissue paper on blocking micro-droplet sized starch particles in a sneeze was about 4.5-7.0 m/s (1, 2). Face masks, such
(average 8.2 μm) and latex microspheres (0.75 μm) with a as surgical masks could block micro-droplets (>5 μm) and
velocity of 44.4 m/s created by centrifugation was aerosols (
in vivo 34: 1637-1644 (2020)
Figure 1. Continued
sneezing or coughing. This study presents a simple way to Mask washing method. Both gauze masks and surgical masks were
test the filter performance of mask materials using micro- soaked in water with a neutral detergent for 10 min. After rinsing
with clean water 3-5 times, the masks were soaked in 1.5% chlorine
droplets sized particles and microspheres with a challenged
bleach for another 10 min. Then the masks were rinsed with clean
velocity of 44.4 m/s created by centrifugation (7,500 rpm). water 3-5 times and air dried.
Materials and Methods Starch solution preparation. One tea spoon of soft wheat flour
(commercially available) was mixed with food coloring in 50 ml of
Samples of mask materials. Samples included surgical masks, water. The solution was further completely mixed by voltex before
washed surgical masks, cotton gauze masks (containing 4 layers of being added onto each sample.
cotton gauze, 2 layers of nonwoven fabric filter and 2 layers of
polypropylene filter, washed), 4 types of cotton from T-shirts, 3 Microsphere solution preparation. One droplet of Fluoresbrite® YG
types of silk, 3 types of linen, tissue paper and cotton gauze. Carboxylate Microspheres 0.75 μm (Cat#07766-10, Polysciences,
Surgical masks and cotton gauze masks were purchased from a Inc., Warrington, PA, USA) was added into 50 ml of water to make
Japanese drug store. Both of them have 99% BFE. The surgical an aqueous suspension. The solution was completely mixed by
masks also passed PFE and VFE tests. The microstructures of mask voltex before being added onto each sample.
materials were observed by both a digital microscope (AM4113ZT4,
ANMO Electronics Corp., Taiwan) and a scanning electron Micro-particle filtration efficiency test. a) Starch particle filtration
microscope (SEM) (S-4000, HITACHI, Ltd. Tokyo). efficiency test. 300 μl of starch solution was gently dropped on the
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Xiao et al: A New Method for Mask Evaluation
Figure 1. Microstructures of mask materials. A. Polarized microscopic images of mask materials including 4 types of 100% cotton from T-shirts,
surgical mask surface, surgical mask filter, gauze mask nonwoven fabric filter, gauze mask polypropylene filter, 3 types of linen, 3 types of silk. (i),
commercial available cotton gauze mask, (ii) macrostructure of the cotton gauze mask. B. SEM images of some mask materials.
top of each sample which was set in the insert of a micro-tube intensity of each droplet was then analyzed by ImageJ (version
[Figure 2(i)] and followed by centrifugation (7,500 rpm, equaling 1.52u, Wayne Rasband, National Institutes of Health, Bethesda,
44.4 m/s) for 20 s to mimic the velocity of a sneeze. After MD, USA).
centrifugation, the solution was mixed completely by pipetting. For c) Microsphere-capturing test. 50-100 μl of microsphere solution
the hydrophobic samples (surgical masks) or samples with thick was added on top of the sample to make the materials wet throughout
layers, 100 μl of the solution was applied on to the samples and all the layers. After air-drying overnight, samples were mounted onto
centrifuged 6-7 s. This process was repeated three times. 25 μl of stubs and then coated with a thin layer of osmium in the osmium
the filtered solution was added onto a Tali™ Cellular Analysis Slide plasma coater (Neo osmium coater Neoc-AN, Meiwafosis Co., Ltd.
(T10794, Thermo Fisher Scientific, Tokyo) and analyzed by the Tokyo, Japan). The samples were examined using a scanning electron
Tali™ Image Cytometer (Thermo Fisher Scientific, Tokyo). microscope (S-4000, HITACHI, Ltd. Tokyo, Japan).
b) Microsphere filtration efficiency test. 100 μl of microsphere
solution was added to surface of the samples and centrifuged in the Statistical analysis. Statistical analysis was carried out similarly to
same manner as described in the starch particle filtration efficiency our previous report (11). All data, expressed as the mean±SD, were
test. The filtered solution was then mixed completely by pipetting. analyzed statistically by GNU PSPP Statistical Analysis Software
A 1 μl droplet of the solution was added onto a microscope glass (version 0.8.2-gad9374) (https://www.gnu.org/software/pspp/) and
slide and photographed with the EVOS® FL Cell Imaging System EZAnalyze Excel-based tools (http://www.ezanalyze.com/). One-
(Thermo Fisher Scientific, Tokyo, Japan). The green fluorescence way analysis of the variance was followed by post hoc analysis
1639
in vivo 34: 1637-1644 (2020) Figure 2. Efficiency of the mask materials for filtering micro-droplet-sized particles. The experimental steps are illustrated in A. (i) shows the setting of samples. B. Efficiency of mask materials on blocking starch particles was analyzed by Tali cytometer. The histograms represent three independent experiments. **p
Xiao et al: A New Method for Mask Evaluation Figure 3. Efficiency of the mask materials for filtering aerosol-sized particles. The experimental steps are illustrated in A. (i) shows 400 times enlarged image of Fluoresbrite® YG Carboxylate Microspheres with a dimeter of 0.75 μm. Scale=50 μm. B. Efficiency of mask materials in blocking microspheres was analyzed by EVOS® FL Cell Imaging System and ImageJ software. The histograms represent three independent experiments. ***p
in vivo 34: 1637-1644 (2020) Figure 4. Microsphere-capturing ability of mask materials. The experimental steps are illustrated in A. (i) presents a SEM image of the microspheres on a hydrophilic PVDF membrane filter (0.22 μm pore size), scale bar=2 μm. B. SEM images of the microspheres captured by the mask materials (shown by red arrows). green fluorescence. The green fluorescence intensity level of is electrostatically charged, the 4 layers of silk showed the the spots represents the number of microspheres existing in highest blocking rate (93.8%) of the microspheres. Cotton the 1 μl solution. Because these microspheres were too small and linen with multiple layers were significantly better than to measure by the Tali cytometer or flow cytometry, we a single or fewer layers. Washed surgical masks showed analyzed the green fluorescence intensity of each spot to lower filtration efficiency than the new surgical masks. quantify the microspheres blocking rate of mask materials using the ImageJ software. As shown in Figure 3B and C, all Microsphere-capturing ability of mask materials. Lastly, we the mask materials could block the microspheres at different tested the microsphere-capturing ability of materials without degrees. The top three microspheres blocking materials were centrifugation by using SEM (Figure 4). This experiment the 4 layers of silk, the gauze mask and the 2 layers of simulated a situation in which the micro-droplets are spewed cotton-1 (cotton-1 showed average filtration efficiency onto the surface of a mask. Figure 4(i) showed that the among cotton1-4 in the starch particles test, therefore we microspheres did not penetrate the 0.22 μm filter. Because chose it for this experiment). They all showed significantly the surface of surgical masks is hydrophobic, 50 μl of higher blocking ability than the surgical mask. Because silk microspheres did not pass through the surface without 1642
Xiao et al: A New Method for Mask Evaluation
centrifugation. The microspheres accumulated on the fibers outer layers of a surgical mask for 4-7 days whereas they
of the mask surface, and were not seen in the third layer. could not be detected on tissue paper after 3 h (15).
Meanwhile, the solution of microspheres penetrated the Therefore, putting several layers of tissue paper inside the
layers of silk, the cotton and the linen samples. Microspheres surgical mask could offer more protection than a surgical
were observed both on their first and last layers, suggesting mask alone. Washed surgical masks lost their
that these materials could capture those microspheres. Notice hydrophobicity and static electricity. They showed lower
that some microspheres presented on the edge of the silk blocking rates of starch particles and microspheres.
fibers suggesting that silk fibers probably electrostatically Previous studies showed that cotton fabric could reduce
captured those microspheres. virus titer (16, 17) suggesting that cotton is a suitable
material for masks. Linen has been used in hospital textiles
Discussion due to its ability to disperse heat and evaporate moisture.
Our data showed that 2 layers of linen could block 53.2% of
Previous studies suggested that the filtration efficiency of the microspheres and 80.3% of the starch particles
masks is basically affected by particle size and filtration suggesting that it has the ability to block some micro-
velocity (14). Centrifugation can easily produce enough droplets. Taken together, we recommend people to make
force to separate the particles inside a solution at high masks with 1) 2 layers of 100% cotton from T-shirts, 2) 4
velocities. As we demonstrate in Figure 2(i), with an insert layers of silk or 3) a combination of linen/cotton or silk/linen
placed in the tube, it is simple to check the filtration or silk/cotton (using a wire around the nose could make the
efficiency of materials by centrifugation. Unlike direct force masks fit well). However, none of these materials could
produced by negative pressure, this method does not break block particles totally, and even a small percentage of the
the fibers of the testing materials even at a challenged micro-droplets and aerosols that passed through the masks is
velocity. The mask materials we used in this study were enough to cause infection. Therefore keeping a social
similar to a previous study in which the researchers distance, avoiding crowds and frequently airing rooms are
evaluated the capacity of homemade masks to block very important.
bacterial and viral aerosols by using a fit test machine. Their However, this study has some limitations. Centrifugation-
results showed that 100% cotton from T-shirts was the best produced velocity only partially represents the velocity of a
choice for homemade masks although its efficiency in cough or a sneeze. The dynamics of particles in fluid and in
blocking transmission was 3 times lower than that of air are also different. It would be best if this study had used
surgical masks (9). A recent study also checked the filtration human subjects to test the filtration efficiency of masks
efficiency of common fabrics at two different airflows: ~35 while coughing and sneezing. It has been reported that
L/min and ~90 L/min. The data showed that some cotton digital high-vision and high-speed video systems could
fabrics and the 4 layers of silk exhibited better or equal quantitatively analyze micro-droplets while coughing or
efficiency to that of N95 or surgical masks on blocking sneezing (18, 19). This technique could be applied for testing
aerosol particles (10 nm to 6.0 μm). The filtration the filtration efficiency of masks.
efficiencies were improved by the increased TPI or layers
of fabrics (10). Our data showed similar results; the 4 layers Conclusion
of silk, the cotton gauze masks, and the 2 layers of cotton
from T-shirts could efficiently block 0.75 μm microspheres To our knowledge, this is the first report to use
and 8.2 μm starch particles under a sneeze-like pressure. centrifugation to test the filtration efficiency of mask
Their blocking efficiency was significantly better than the materials under sneeze-like pressure. This method not only
surgical masks. Evidence showed that surgical masks could compensates for shortcomings of PFE, VFE and BFE tests,
not efficiently filter particles under a size of 3.1 μm (4). but also offers a simple way to explore new materials for
These phenomena possibly occur because the intervals of manufacturing masks during pandemics.
fibers in surgical masks are much looser than in cotton and
silk (Figure 1B and 4B); micro-particles can easily pass Conflicts of Interest
through the surgical mask layers at the velocity of a sneeze.
The Authors declare no competing financial interests regarding this
However, since surgical masks have hydrophobic and
study.
electrocharged fibers, they can offer fluid resistance and
electrostatically capture micro-particles. Interestingly our
Authors’ Contributions
data showed that the 4 layers of tissue paper could block
84.4% of starch particles, which was significantly more L. X. designed and performed experiments, analyzed data and wrote
efficient than surgical masks. It has been reported that the paper; H. S analyzed data and gave conceptual advices; N.M.
infectious SARS-CoV-2 could be detected on the inner and provided materials, analyzed data and proofread the manuscript.
1643in vivo 34: 1637-1644 (2020)
Acknowledgements 9 Davies A, Thompson KA, Giri K, Kafatos G, Walker J and Bennett
A: Testing the efficacy of homemade masks: would they protect in
The present study was supported in part by a Grant-in-Aid for an influenza pandemic? Disaster Med Public Health Prep 7(4):
AntiAging Scientific Research #1517 by Japanese Center for 413-418, 2013. PMID: 24229526. DOI: 10.1017/dmp.2013.43
AntiAging MedSciences which was authenticated by Hiroshima 10 Konda A, Prakash A, Moss GA, Schmoldt M, Grant GD and
Prefecture Government as a non-profitable organization corporation, Guha S: Aerosol filtration efficiency of common fabrics used in
Hiroshima, Japan to XL. The author would like to thank Fusako respiratory cloth masks. ACS Nano acsnano.0c03252, 2020.
Mitsuhashi for her technical support. The authors also appreciate PMID: 32329337. DOI: 10.1021/acsnano.0c03252
Nathaniel Green’s proofreading. 11 Xiao L, Saiki C and Okamura H: Oxidative stress-tolerant stem
cells from human exfoliated deciduous teeth decrease hydrogen
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